Disc drives are commonly used in workstations, personal computers, laptops and other computer systems to store and retrieve vast amounts of user data. A typical disc drive comprises a plurality of magnetic discs that are rotated by a spindle motor at a constant high speed. The surface of each disc is divided into a series of data tracks which are spaced radially from one another across a band having an inner diameter and an outer diameter. The data tracks extend circumferentially around the discs and store data in the form of magnetic flux transitions within the radial extent of the tracks on the disc surfaces.
Each data track is divided into a number of data sectors that store fixed sized blocks of user data. Embedded among the sectors on each track are servo fields that enable the disc drive to control the position of heads used to transfer the user data between the discs and a host computer. More particularly, the heads are mounted to a rotary actuator assembly which includes a coil of a voice coil motor, so that the position of the heads relative to the tracks can be maintained by the application of current to the coil by a closed loop digital servo system in response to the servo information read by the servo fields. For an example of a typical digital servo system, see U.S. Pat. No. 5,262,907 issued Nov. 16, 1993 to Duffy et al., assigned to the assignee of the present invention.
The servo fields are written to the discs during the manufacture of a disc drive using a highly precise servo track writer, which utilizes the heads of the disc drive to write the servo fields. As the servo fields are used to define the tracks, it is important to precisely control the position of the heads as the servo fields are written to the disc surfaces. Thus, a typical servo track writer comprises a positioning system which advances the position of the heads, a laser based position detector which detects the position of the heads and control circuitry which provides the servo information to be written to the servo fields on the discs.
In one common type of servo track writer, the positioning system includes a pusher pin assembly that engages the actuator assembly through an opening in the disc drive base deck, in which case the position detector detects the position of the heads by detecting the radial position of the pusher pin assembly. In another common type of servo track writer, the positioning system controls the position of the heads directly by applying current to the coil of the disc drive voice coil motor, in which case the position detector detects the position of the heads by observing the radial position of the actuator assembly. For reference, servo track writing methodologies are generally discussed in U.S. Pat. No. 5,164,863 issued Nov. 17, 1992 to Janz and U.S. Pat. No. 5,241,430 issued Aug. 31, 1993 to Janz, both of which are assigned to the assignee of the present invention.
As will be recognized, proper radial alignment of the servo fields is essential to facilitate reliable operation of the disc drive. For example, when errors are introduced in the placement of the servo fields, components at corresponding frequencies can appear in a position error signal (PES) generated by the servo system during subsequent operation of the drive. The PES is a measure of the relative position of a selected head with respect to an associated track and is used primarily during track following operations to maintain the head over the center of the track. Thus, such frequency components appearing in the PES for a selected track will result in the repeated adjustment of the position of the head by the servo system in an attempt to maintain the head over the center of the track during each revolution of the disc. When such frequencies are sufficiently severe, the correction required to account for these frequencies can require a significant amount of the total track misregistration budget, limiting the overall track density that can be achieved in a disc drive design.
It is well known that the excitation of system resonances of the servo track writer can result in oscillations at the heads, leading to errors in the placement of the servo fields and corresponding frequency components in the PES during subsequent disc drive operation. System resonances can be excited from, for example, vibrations generated by the operation of the disc drive spindle motor during rotation of the discs as the servo fields are written. Particularly, ball bearings of the spindle motor can generate characteristic frequencies of vibration which are typically a function of the rotational speed of the discs.
Attempts to minimize the effects of system resonances have included efforts to adjust the mechanical response characteristics of servo track writers and to select a rotational speed (in revolutions per minute, or RPM) for a population of nominally identical disc drives that tends to minimize the excitation of system resonances during the servo track writing operation. However, such efforts generally have been found to be increasingly deficient for successive generations of disc drives. That is, as disc drive track densities increase, greater demands are placed upon servo track writers to accurately locate the servo fields on the discs; thus, vibration levels that were acceptable for earlier generations become increasingly unacceptable for later generations of drives. Moreover, the complexity and accuracy of servo track writers has increased in order to accommodate the writing of higher track densities; however, certain of these newer models have been found to have characteristic resonances that are excited by so many different spindle bearing frequencies that it is difficult to specify an acceptable rotational speed for any given population of disc drives.
Accordingly, there is a need for an improved approach to reducing the system resonances in a disc drive servo track writer in order to reduce or eliminate the effects of frequency components in a PES generated from the servo fields during subsequent disc drive operation.